This invention relates to electrostatographic imaging systems and more specifically to improved carrier compositions useful in the development of electrophotographic images.
It is well known to form and develop images on the surface of photoconductive materials by electrostatic means, one of the more basic systems being described in C. F. Carlson U.S. Pat. No. 2,297,691. This process is also described in other U.S. patents including, for example, U.S. Pat. No. 2,227,013; U.S. Pat. No. 2,357,809; U.S. Pat. No. 2,551,582; U.S. Pat. No. 3,220,324 and U.S. Pat. No. 3,220,833. The processes described in these patents generally involve the forming of an electrostatic charge latent image on an insulating electrophotographic element whereby the latent image is made visible by a development step wherein the charged surface of the photoconductive element is brought into contact with a suitable developer mix. As described in U.S. Pat. No. 2,297,691, for example, the resulting electrostatic latent image is developed by depositing on the image a finely-divided electroscopic material referred to in the art as toner. This toner is generally attracted to the areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image and subsequently the toner image can be transferred to a support surface such as paper. This transferred image can then be permanently fixed to the support surface by using a variety of techniques including heat; however, other suitable fixing methods such as solvent or overcoating treatment may be used.
Numerous methods are known for applying the electroscopic particles to the electrostatic latent image including cascade development, touchdown and magnetic brush belt. In cascade development, as described in U.S. Pat. No. 2,618,552, a developer material comprising relatively large carrier particles having finely-divided toner particles electrostatically clinging to the surface of the carrier particles is conveyed to and rolled or cascaded across the electrostatic latent image-bearing surface. The composition of the toner particles is selected in order to have a triboelectric polarity opposite to that of the carrier particles. Thus, as the mixture cascades or rolls across the image-bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of the latent image and are not deposited on the uncharged or background portions of the image. Carrier particles and unused toner particles can then be recycled. This process is fully described by E. N. Wise in U.S. Pat. No. 2,618,552.
In magnetic brush development, a developer material comprising toner and magnetic carrier particles is carried by a magnet whereby the magnetic field of the magnet causes alignment of the magnetic carriers in a brush-like configuration, and this brush is engaged with an electrostatic latent image-bearing surface, causing the toner particles to be attracted from the brush to the electrostatic latent image by electrostatic attraction. This process is described more fully in U.S. Pat. No. 2,874,063.
Some recent efforts to improve copy quality have focused on the carrier particles, and more specifically the coating of those particles in order to obtain a better development system, particularly a developer that can be recycled and does not cause injury to the photoconductor. The importance of carrier coating compositions takes an increased emphasis in different development techniques. Generally, in order to develop a latent image comprised of negative electrostatic charges, an electroscopic powder and carrier combination is selected in which the toner powder is triboelectrically positive relative to the granular carrier. To develop a latent image comprised of positive electrostatic charges such as when employing a selenium photoreceptor, an electroscopic powder and carrier mixture is selected in which the powder is triboelectrically negative relative to the carrier. However, where the latent image is formed of negative electrostatic charges, such as when employing organic electrophotosensitive materials as the photoreceptor, it is desirable to develop the latent image with a positively charged electroscopic powder and a negatively charged carrier material. Thus, there is a continuing need for a better developer material for developing electrostatic latent images.